Dozens of migrants were found dead in the back of a truck in the southern US state of Texas on Monday, local officials said.

The sweltering semi-trailer was discovered on the outskirts of San Antonio, around 160 miles (250 kilometers) from the US southern border with Mexico.

The vehicle was found near US interstate 35, a major US highway that leads to the Mexican border.

On Tuesday, Mexican President Andres Manuel Lopez Obrador said the total number of dead was 50, among them 22 Mexicans. Local officials originally said Monday that 46 people had been found dead.

On Tuesday, officials at a San Antonio hospital said 2 more people had died after treatment was unsuccessful, putting the death toll at 48. There was no immediate reason given for the discrepancy between the US and Mexican numbers. 

"We're not supposed to open up a truck and see stacks of bodies in there," San Antonio fire chief Charles Hood said Monday.

How were the bodies found?

A worker in a nearby building had been alerted to the situation after hearing a cry for help on Monday afternoon, said San Antonio police chief William McManus.

Police officers said they discovered the bodies, along with another 12 adults and four minors who were found alive inside the truck. 

They were rushed to the hospital with heat-related illnesses. Among the survivors were two Guatemalans, Mexican officials said.

Three people have been taken into custody, according to McManus, but their connection to the deaths remained unclear.

Fire chief Hood said the individuals were hot to the touch and suffering from heat stroke. No water or air conditioning was found inside the truck.

Temperatures in San Antonio reached a high of 39.4 degrees Celsius (103 degrees Fahrenheit) on Monday when the truck was found.

The temperature inside a trailer filled with people is typically even higher than the outside temperature.

An investigation is underway into the incident and three individuals are in custody. None of those three were found inside the truck.

Government authorities in the US have not yet commented on the nationalities of the victims.

Mexico's Foreign Minister Marcelo Ebrard said on Twitter that there had been a discussion with the country's attorney general, and an investigation would be opened "due to the events related to our territory."

Officials offer condolences, blame

In a statement Tuesday, the White House called the immigrant deaths "absolutely horrific and heartbreaking." 

US President Joe Biden took aim at criminal syndicates involved in smuggling operations and said that "exploiting vulnerable individuals for profit is shameful." Biden added that his administration was working to clamp down on criminal networks.

San Antonio Mayor Ron Nirenberg addressed reporters at the scene. "This is nothing short of a horrific human tragedy," he said.

The nearby Mexican consulate also dispatched officials to where the truck was found, with Mexican Foreign Minister Ebrard calling it "the tragedy in Texas."

"Condolences to the victims and their families," he said. 

kb,zc,rs,es/rs (AP, Reuters)